Method and apparatus for measuring a thickness of a nonconductive coating and calibrating a thickness measurement gauge

ABSTRACT

A method and apparatus are provided for measuring a thickness of a nonconductive coating disposed over portions of first and second conductive surfaces that intersect at an intersection angle. The apparatus is a thickness measurement gauge having an eddy current sensor. The thickness measurement gauge includes an eddy current sensor and electronic analyzer. The thickness gauge may be provided with a pressure enclosure. A method of calibrating a thickness measurement gauge and a calibration stand are also provided. The calibration stand has third and fourth conductive surfaces intersecting at the intersection angle. The conductivities of the third and fourth surfaces correspond to the conductivities of the first and second surfaces.

FIELD OF THE INVENTION

This invention relates to coating thickness measurement gauges, and,more particularly to an eddy current thickness gauge for measuringnonconductive coatings, a method of measuring a nonconductive coating atan intersection with an eddy current thickness gauge, and methods andapparatuses for calibrating an eddy current thickness gauge.

BACKGROUND OF THE INVENTION

Nonconductive coatings are applied at intersections of materials,typically in the form of a fillet seal for purposes of enclosing andsealing the intersection. Often these coatings are required to attain aminimum thickness. In many cases, these coatings can shrink by as muchas 20% as they dry. As such, what appears to be an adequate coatingthickness may later shrink to less than a minimum thickness. In theaerospace industry, quality assurance programs have been developed toinspect coatings and fillet seals on aircraft to ensure that theyachieve minimum desired thicknesses.

For example, one sealant, BMS5-26, Type II, Integral Fuel Tank Sealant,is used to coat intersections of an airplane wing. In many aircraft, thewing is also the support structure for an internal bladder fuel tank,and it is therefore important to seal intersections and gaps. Thesealant must be applied to any intersection to prevent a leak path. Thesealant is typically applied to the border of all electricalpass-through brackets, all fasteners, and along the faying surface ofthe web and skin. The greatest amount of sealant is applied along theintersection of the heavy longerons and the wing skin.

When inspecting the sealant coatings at these intersections, qualityassurance inspectors attempt to determine the minimum thickness of acoating. Unless coupled with a destructive test, nondestructive visualinspections are often not adequate. One test requires removing thesealant and physically measuring the thickness, from the radius of aconcave fillet to the intersection of the web and skin. The physicalmeasurement of the destroyed seal may then provide a rough visualreference for performing a visual inspection of the remaining sealant.These tests are based upon the visual perception of the inspector.

This procedure has several drawbacks. For one, visual inspectionsintroduce subjective reproducibility errors. Also, the destroyed sectionmust be repaired by having a coating seal reapplied. Consequently, therepaired area is not as structurally contiguous as the original seal.Also, additional labor is required to perform the destructive test andsubsequently reapply the sealant. In the case of hard to reach areas,visual assessment is made based on experience. When an inspector is notconvinced that a proper amount of sealant has been applied, they mayinsist that sealant be reapplied. Reapplication of sealant, therefore,also increases labor.

As such, there has not been an adequate non-destructive method fordetermining minimum coating thickness requirements. Therefore, there isa need in the art for a portable nondestructive coating thicknessmeasurement gauge capable of measuring coating thicknesses atintersections, such as a fillet seal.

BRIEF SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a method ofmeasuring a thickness of a nonconductive coating is provided. Thecoating is applied over portions of a first surface and a second surfacethat intersect at an intersection angle. The method measures the coatingwith a thickness measurement gauge having an eddy current sensor. Onestep of the method comprises placing the eddy current sensor in contactwith at least a portion of the coating at a predetermined angularorientation with respect to the first and second surfaces. The angularorientation is non-perpendicular and non-parallel to both the first andsecond surfaces. The method also includes determining a measurementoutput of the thickness measurement gauge, which is based on the firstand second conductivities of the first and second surfaces,respectively. From the output, the method includes determining athickness of the nonconductive coating.

One aspect of the step of determining a thickness further comprisescomparing the measurement output to a predetermined output. Thiscomparison may be a pass/fail tolerance corresponding to the differencebetween the measurement output and the predetermined output.

Another embodiment of the present invention includes a method ofcalibrating a thickness measurement gauge having an eddy current sensor.The eddy current sensor may be used to measure a thickness of anonconductive coating disposed over a first surface and a second surfacethat intersect at an intersection angle. The first and second surfaceshave first and second conductivities, respectively. The method includesplacing the eddy current sensor at a predetermined angular orientationwith respect to third and fourth surfaces that intersect at the sameintersection angle. The third and fourth surfaces have third and fourthconductivities corresponding to the first and second conductivities,respectively. The method includes positioning the eddy current sensor ata predetermined distance with respect to the third and fourth surfacesand calibrating to a predetermined output based on the third and fourthconductivities.

An aspect of the method of calibration includes inserting anonconductive coating at the intersection of third and fourth surfacesprior to performing the step of calibrating. The nonconductive coatingmay, in some cases, include a nonconductive coating comprised of thesame material as the nonconductive coating between the first and secondsurfaces. Additionally, the nonconductive coating may comprise a coatingof a thickness corresponding to a minimum desired coating thickness forthe coating on the first and second surfaces. Another aspect of themethod of calibrating includes positioning the eddy current sensor incontact with at least a portion of the nonconductive coating.

Another embodiment of the present invention comprises a thicknessmeasurement gauge. The thickness measurement gauge includes a sensorhousing defining a first sidewall on the exterior of the housing. Thefirst sidewall permits the housing to abut a first surface proximate toa measurement area. An eddy current sensor is disposed in the housingand defines a longitudinal axis of measurement corresponding to magneticfields generated by the eddy current sensor. Generally, the axis ofmeasurement is at a non-perpendicular angle to the first sidewall.

Various aspects of the thickness measurement gauge include a housinghaving a second sidewall that permits the housing to abut a secondsurface proximate to the measurement area. As such, the eddy currentsensor is disposed between first and second sidewalls such that the axisof measurement is at a non-perpendicular angle to the second sidewall.In one embodiment, the eddy current sensor is disposed between the firstand second sidewalls such that the axis of measurement is at a similarangle with respect to both first and second sidewalls. As used herein,“similar” is used in its mathematical sense designating correspondingangles that are equal. In one case, the first and second sidewalls areperpendicular to each other and permit the sidewalls to abutperpendicular intersection surfaces.

The eddy current sensor may be electrically interconnected to anelectronic analyzer. The electronic analyzer is adapted to analyze anelectrical signal from the eddy current sensor corresponding to adistance from a conductive surface. According to one aspect of themeasurement gauge, the electronic analyzer is independent of the housingand permits the sensor and sensor housing to be placed in narrow areas.The electronic analyzer may also further comprise a calibration circuit.For example, several commercially available electronic analyzers includelinearity, zero crossing, and gain adjustments to permit the output ofthe electronic analyzer to be adjusted to a predetermined output.

Another embodiment of a thickness measurement gauge includes a pressureenclosure housing an electronic analyzer. The pressure enclosure ishermetically sealed to permit a pressure within the enclosure to exceedambient pressure. An eddy current sensor is disposed in a housing andelectrically interconnected through the pressure enclosure to theelectronic analyzer. As such, an explosive resistant thicknessmeasurement gauge is provided.

The pressure enclosure may also include an electrical connectorhermetically sealed about the pressure enclosure to permit connectionbetween the electronic analyzer and sensor by way of a lead. Otherconnectors through the enclosure are similarly hermetically sealed andmay include other electrical connectors and also pneumatic connectors.For example, valve connections, pressure relief, pressurizationconnections, and display connections through the pressure enclosure, ifprovided, may include hermetic seal to permit a positive pressure insidethe enclosure. In one case, one pneumatic connection is a Schrader valvethat permits the enclosure to be pressurized by commonly available airhoses and pumps.

Other aspects of an embodiment having a pressure enclosure may include apower source for energizing the electronic analyzer, such as a batterydisposed within the pressure enclosure. This embodiment thereforepermits a pressure switch to be electrically interconnected between thepower source and the electronic analyzer. As such, the pressure switchcan disconnect the electronic analyzer and power source upon pressurewithin the pressure enclosure falling below a predetermined threshold.Therefore, the explosion resistance of the pressure enclosure ismaintained.

Another embodiment of the present invention includes a calibration standfor calibrating a thickness measurement gauge including an eddy currentsensor. The eddy current sensor is calibrated to measure a thickness ofa non-conductive coating disposed over portions of a first and secondsurface that intersect at an intersection angle. The first and secondsurfaces have first and second conductivities, respectively. Thecalibration stand includes an eddy current sensor holder. Opposed to theeddy current sensor holder is a calibration block having third andfourth surfaces intersecting at the same intersection angle. The thirdand fourth surfaces lie at a predetermined angular orientation withrespect to the sensor holder. The third surface has a third conductivitycorresponding to the first conductivity, and the fourth surface has afourth conductivity corresponding to the second conductivity. Apositioner linearly translates the eddy current sensor holder toward andaway from the calibration block, and may include a micrometer gaugecorresponding to the linear distance between the sensor and thecalibration block.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is cross section of a thickness measurement gauge abutting firstand second surfaces according to one embodiment of the presentinvention;

FIG. 2 is a block diagram of the operation of a thickness measurementgauge according to one embodiment of the present invention;

FIG. 3 is a perspective of a thickness measurement gauge pressureenclosure according to one embodiment of the present invention;

FIG. 4 is an exploded view of a thickness measurement gauge pressureenclosure according to one embodiment of the present invention;

FIG. 5 is a block diagram of the operation of a pressure enclosure andthickness measurement gauge according to one embodiment of the presentinvention;

FIG. 6 is a perspective view illustrating the pneumatic connections ofthe pressure enclosure according to one embodiment of the presentinvention;

FIG. 7 is a calibration block for a thickness measurement gaugeaccording to one embodiment of the present invention; and

FIG. 8 is a thickness gauge positioned relative to a v-block forcalibrating the thickness measurement gauge according to one embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring to FIG. 1 and according to one embodiment of a thicknessmeasurement gauge 10, an eddy current sensor 12 is disposed within asensor housing 14. The eddy current sensor 12 is typically used tomeasure coating thicknesses. One application of the thicknessmeasurement gauge 10 includes measuring a nonconductive coating 24,covering a portion of the intersection of two conductive surfaces 20, 22such as a fillet seal depicted in FIG. 1. While the followingembodiments are described in conjunction with a fillet seal, theinvention may be used to measure the thickness of any nonconductivecoating over any of two intersecting conductive surfaces and is notlimited to sealants or sealing materials.

The sensor housing 14 also comprises at least one sidewall 16, and, inthis embodiment, a first sidewall 16 and a second sidewall 18. The firstand second sidewalls 16, 18 permit the sensor housing 14 to be placedproximate to a measurement area at the intersection of first and secondconductive surfaces 20, 22. As such, the first sidewall 16 may abut thefirst conductive surface 20, and the second sidewall 18 may abut thesecond conductive surface 22. Alternatively, the sidewalls may permitthe sensor housing to abut other surfaces proximate the measurement areaso long as the angular orientation of the sensor with respect to theintersection is maintained. The angular orientation is discussed in moredetail below.

The eddy current sensor 12 is disposed along a measurement axis 19,which is non-parallel to and at non-perpendicular angles to first andsecond sidewalls 16, 18. The measurement axis 19 corresponds to themajor axis of the magnetic field density generated by the eddy currentsensor 12. The eddy current sensor 12 generates a magnetic field thatinduces an eddy current in the first and second conductive surfaces 20,22. The eddy currents intensities are related to the conductivities ofthe surfaces 20, 22 and the distance from which the eddy current sensoris positioned away from each surface. The eddy currents induced in firstand second conductive surfaces 16, 18, therefore, produce an opposingmagnetic field, which are sensed by a magnetic coil on the sensor.Because of the generated magnetic fields, it is also advantageous thatthe sensor housing 14 be made of a nonconductive material in order toadequately prevent interfering eddy currents and magnetic fields.

It has been determined that the resultant magnetic fields (a vector sumof the original magnetic field and the opposed magnetic fields generatedby the eddy currents), and thus the output from the sensor 12, varyaccording to the distance of the eddy current sensor 12 to theintersection of the first and second conductive surfaces 20, 22. Over arange of distance from the intersection, it is possible to calibrate theeddy current sensor 12 and associated circuitry to linearly approximatethe output on the sensor 12, that is, to have an output that varieslinearly with the distance to the intersection. To this end, thethickness measurement gauge 10 of this embodiment may be used todetermine the distance from the intersection to the eddy current sensor12. In practice, this may be used to determine the thickness 26 of anonconductive coating 24, such as a sealant, placed over theintersection. It should be noted that the conductivities of the firstand second conductive surfaces 20, 22 need not necessarily be the same,however, it is expected that the conductivities will be within a rangeof each other so that the conductivity of one conductive surface is noteffectively negligible compared to the other conductive surface.

One embodiment of a sensor 12 that may be disposed within the sensorhousing is a Kaman 9C manufactured by the Kaman Corporation, KamanInstrumentation Operations, Measuring Systems Group, Colorado Springs,Colo. An electronic analyzer, the Kaman KDM-8200 circuit board, controlsthe 9C sensor. The operation, set-up, and calibration of the sensor andKaman KDM-8200 are further explained in KDM-8200 Instruction Manual,Part No. 860059-001 Rev. F, the teachings of which are herebyincorporated by reference. While the embodiments herein illustrate anelectronic analyzer and sensor consistent with the Kaman 9C andKDM-8200, many other eddy current sensor and associated electronicanalyzers are available and may be substituted accordingly.

Referring to FIG. 2 and for purposes of illustration, the Kaman 9Celectronic analyzer 40 includes a sensor coil 32, and KDM-8200 includesa bridge network 42, synchronous demodulator 44, log amp 46, and outputamp 48. An AC current is provided to the sensor coil 32 by the balancedbridge network 42. As a result, the sensor coil 32 generates a magneticfield. The coil 32 is positioned such that the longitudinal axis of thesensor corresponds to the measurement axis 19 from FIG. 1. When placedat a distance from two intersecting conductive surfaces 20, 22, themagnetic field induces eddy current in each surface. The eddy current inthe conductive surfaces 20, 22 generates opposing magnetic fields,reducing the intensity of the original magnetic field. Therefore, theimpedance of the sensor coil 32 is varied and output voltage isproportional to the distance from the intersection of the conductivesurfaces 20, 22.

The sensor coil 32 is controlled by the balanced bridge network 42adapted to sense the change in impedance. About a range of distance fromthe intersection of the first and second surfaces 16, 18, the balancedbridge network 42 may be calibrated so that the sensor coil impedancerepresents a linear approximation of the distance. The electronicanalyzer 40 includes signal conditioning components, such as asynchronous demodulator 44, log amp converter 46, and output amplifier48 that permit calibration of the sensor to a desired linearity orslope, gain, and zero crossing as is known to one of ordinary skill inthe art. This embodiment is illustrative of the general functions of theKDM-8200, however, other electronic analyzers are known to those ofordinary skill in the art and may be substituted accordingly.

In many embodiments, a coating need only attain a minimum thickness, andthe thickness measurement gauge may be used with a pass/fail test toascertain whether the thickness 26 achieves the minimum thickness. Forexample, the thickness gauge 10 may be calibrated to a desired outputcorresponding to a distance of the sensor 12 from the intersection ofthe conductive surfaces 16, 18 corresponding to a minimum thickness.When the sensor 12 is placed proximate the intersection of first andsecond surfaces 16, 18 and in contact with a portion of the coating 24,the output may be compared to the calibrated output for a predetermineddistance. For an output with a positive slope with respect to distance,any reading less than the calibrated reading would suggest the thickness26 is less than the required minimum, and thus a fail. Otherwise thetest is a pass since the thickness would be more than the predefinedacceptable minimum. Additional methods of determining thickness 26 of anonconductive coating are described in more detail below.

Referring back to FIG. 1 and according to this embodiment of a thicknessmeasurement gauge 10, the sensor housing 14 comprises a first sidewall16 disposed on one side of the sensor 12. The first sidewall 16 permitsthe sensor housing to rest against a surface 20 proximate to themeasurement area. At least one sidewall is required of the sensorhousing 14 so that the sensor can be placed against a surface proximatea measurement area and over a portion of the intersection of first andsecond surfaces 20, 22. According to this embodiment, a second sidewall18 is provided so that the sensor holder 14 may be firmly placed betweenboth first and second surfaces 20, 22. The first and second sidewalls16, 18 are disposed on opposed sides of the sensor 12 to permit thesensor housing 14 to abut at least one of the conductive surfaces 20,22, and in this embodiment both first and second conductive surfaces 20,22. In numerous applications, the first and second conductive surfaces20, 22 are often perpendicular to one another, and therefore the firstand second sidewalls 16, 18 of the sensor housing are perpendicular toone another. Other embodiments may permit a variety of angles and thefirst and second sidewalls should be angled in order to effectivelypermit at least one and, more advantageously, both sidewalls to abut asurface proximate a measurement area over a portion of the intersection.

As can be seen in FIG. 1, the first sidewall 16 includes a recessedportion 21 permitting the housing to recede inwardly toward the sensor12. Additionally, the first and second sidewalls may converge at abeveled corner 23 (not shown in the cross section but more fullyillustrated in FIG. 8). The beveled corner is generally parallel to andslightly larger than the end of the sensor 12. The combination of therecessed portion and beveled corner, therefore, permits the sensor 12 tocontact a portion of the coating during measurement. Also, according tothis embodiment, the sensor 12 is disposed between the sidewalls 16, 18such that the measurement axis 19 is disposed at a similar angle to eachsidewalls 16, 18. Having the measurement axis 19 at similar maximize theeffectiveness of the magnetic field along the first and second surfaces20, 22. As used herein, “similar” is used in its mathematical sensedesignating corresponding angles that are equal.

As described in this particular embodiment, the sensor 12 is disposed ina sensor housing 14 and the electronic analyzer 40 is disposedindependent of and remote from the sensor housing. An analyzer, such asthe KDM-8200, is typically disposed on a large circuit board, andtherefore incapable of being inserted into small and confined areaswhere measurements are taken. Consequently, an independent electronicanalyzer 40 permits the sensor housing 14 to be as small as possible tohold the desired sensor and be able to fit in narrow and confinedspaces. In aircraft manufacture, for example, the intersection of a wingskin and longeron provides narrow and difficult to reach areas, and asmaller sensor and sensor housing provide increased capability to reachand measure these areas. As such, the sensor 12 may be interconnected tothe analyzer via a lead 28. As used herein, when a device or element is“interconnected” to another device or element, it may be directlyconnected, attached, or connected by one or more intervening devices orelements. Not all intersections are in confined areas however, andtherefore the electronic analyzer need not be independent of and remotefrom the sensor housing in all embodiments.

Referring concurrently to FIGS. 3 and 4, another embodiment of athickness measurement gauge includes a circuit card 64 having anelectronic analyzer 40, which is disposed in a hermetically sealedpressure enclosure 50. An eddy current sensor, such as the one describedabove, disposed in a housing, may be externally connected through thepressure enclosure 50 via a lead. The pressure enclosure 50 ishermetically sealed to permit a positive air pressure to be maintainedinside. The pressure enclosure 50 also includes associated circuitryincluding a battery 66, and displays 58, 60 on the exterior of theenclosure. The displays 58, 60 and other interconnections through thepressure enclosure 50 are also hermetically sealed.

The hermetically sealed embodiment of a thickness gauge pressureenclosure, therefore, permits the thickness gauge to be used inexplosive environments. For example, in aircraft manufacture, aromaticpolymer sealants (described above in the background section) releaseflammable and explosive vapors. In order to avoid electrical spark inthe area of the sealant, particularly in confined and poorly ventilatedareas such as inside an airplane wing, the hermetically sealed thicknessgauge provides an explosion resistant device. Because of the ability tomaintain a positive pressure, the pressure enclosure 50 prevents anyvapors from the sealant entering through the openings andinterconnections, thus reducing the chance of a spark igniting thevapor. In this particular embodiment, it was found that approximately 4inches of water or 0.15 psi of positive differential air pressure wassufficient for adequate explosion resistance. While air was used in thisembodiment, it is contemplated that inert gasses may be used topressurize the pressure enclosure and may be substituted accordingly.

This embodiment of a thickness gauge having a hermetically sealedpressure enclosure 50 includes a power switch 62, an electronic analyzeroutput display 60, and a pressure gauge 58. The pressure gauge 58provides a measure of the pressure on the inside of the pressureenclosure 50 so that an operator may visually inspect the pressureenclosure 50 to ensure a positive pressure is maintained. The pressureenclosure 50 also includes several pneumatic devices including apressurization valve 52, which in this embodiment is a Schrader valve. ASchrader valve advantageously permits common commercial air pumps topressurize the inside of the pressure enclosure 50. Also included inthis embodiment is a manual pressure relief valve 54 that permitsreleasing the positive pressure from the pressure enclosure.Alternatively, an automatic pressure relief valve may be provided toprevent overpressurization of the pressure enclosure, or both anautomatic and manual relief may be used in combination.

As previously stated, the pressure enclosure 50 includes a circuit board64 containing an electronic analyzer 40, such as the previouslydescribed KDM-8200. FIG. 5 provides a block diagram illustrating theelectronic analyzer 40 disposed in the pressure enclosure 50 a, and likenumbers identifying functional blocks correspond to the physicalillustrations of FIG. 4. Also included within the pressure enclosure 50,50 a is a battery pack 66, 66 a for providing power to the electronicanalyzer 40. According to this embodiment, the battery pack 66, 66 a maybe rechargeable and thus having an interconnection 74 through thepressure enclosure 50, 50 a to allow the battery 66, 66 a to beperiodically recharged. The power switch 62, 62 a is therefore amulti-position switch that permits the battery 66, 66 a to be charged orprovides power to the electronic analyzer 40. While a rechargeablebattery is described in conjunction with one embodiment, it iscontemplated that various other power sources, consistent withelectronic analyzer manufacturer's specifications, may be used includingnon-rechargeable batteries, replaceable batteries, and external AC or DCpower sources without resulting in a change to the basic function of athickness gauge described herein.

A differential pressure bellows 68, 68 a and a differential pressureswitch 70 are also disposed within this embodiment of a thickness gauge.The differential pressure bellows 68, 68 a senses the pressuredifference between the atmosphere external to the pressure enclosure 50,50 a through a pneumatic bulkhead fitting 56, 56 a and internal to thepressure enclosure. The bellows 68, 68 a controls the pressure switch70. The pressure switch 70 is connected in series with the battery pack66 a, power switch 62 a, and electronic analyzer 40. A predeterminedpressure threshold may be established, below which the pressure switchdisconnects power to the electronic analyzer 40. As the pressure in thepressure enclosure drops below the predetermined threshold, there is areduced ability to avoid potentially explosive vapors from entering thepressure enclosure 50. Therefore, the pressure switch provides aninterlock to minimize any explosion hazard while operating the thicknessgauge.

As illustrated in FIG. 6, one embodiment of the pressure enclosure alsoincludes internal tubing 76 interconnecting the various pneumaticdevices. One connection provides the external ambient pressure through apneumatic bulkhead fitting 56 to the differential pressure bellows 68.Similarly, the pressure gauge is provided with a tubing interconnection76 from an external pneumatic bulkhead 56 fitting.

As illustrated in FIG. 7, a calibration stand 80 for calibrating athickness gauge is also provided. In this embodiment the calibrationstand includes a v-block 82 having third and fourth conductive surfaces84, 86. The third and fourth conductive surfaces 84, 86 haveconductivities corresponding to the conductivities of the first andsecond conductive surfaces 20, 22, respectively. As such, the sensordisposed in a sensor housing 14 may be placed in the calibration standso that the axis of measurement is at the same orientation with respectto third and fourth surfaces 84, 86 as the axis of measurement will beoriented to first and second surfaces 20, 22, as previously described.As shown in this embodiment, the v-block 82 is a single piece and thusthird and fourth surfaces 84, 86 have the same conductivity. As is oftenthe case, first and second surfaces 20, 22 may be the same material withequivalent conductivities, and therefore this embodiment of a v-block isadvantageous for calibrating the sensor for first and second surfaces ofsimilar materials. When first and second surfaces 20, 22 comprisedissimilar materials and conductivities, other v-blocks havingdissimilar conductive third and fourth surfaces may be substitutedaccordingly.

According to this embodiment, the calibration stand also comprises asensor holder 99. A flange 98 attached to base 97 supports the sensorholder 99. The sensor holder 99 is set into position within the flange98 by a setscrew 94. Another flange 96 attached to the base 97 supportsthe v-block 82. A setscrew 92 fastens the v-block 82 to the flange 96and a positioning device 88. The positioning device 88 is a linearpositioning screw with micrometer gauge 90 corresponding to the lineardistance of the positioner 88. As such, the v-block 82 is linearlypositioned with respect to the sensor holder 99 by turning thepositioner 88 to the desired distance. It will be appreciated, however,that other positioners may be used to effectively position the sensorhousing 14 and sensor 12 in the desired orientation with respect to thethird and fourth surfaces 84, 86 of the v-block 82 without departingfrom the spirit or scope of the present invention. For example, thepositioner 88 may be adapted on the opposed flange to position thesensor while holding the v-block 82 fast. Other embodiments may usemeasurement devices other than micrometers 90 to effectively assure thedistance and orientation of the sensor holder 99 with respect to thev-block 86. Similarly, support devices other than flanges may be used tosupport a sensor holder and the v-block.

The sensor holder 99 is adapted to hold a sensor housing 14 having aneddy current sensor 12 disposed therein. The sensor holder of thisembodiment is a cylinder having an opening 101 defined on one side. Theopening 101 permits the sensor housing 14 to be securely placed withinthe sensor holder 99 for calibration. As such, this embodiment permitsthe eddy current sensor 12 to be calibrated in a calibration stand 80,such as the one described above, without having to remove the sensor 12from the sensor housing 14. Therefore, this eliminates any potentialdisplacement error associated with replacing the eddy current sensor 12within the sensor housing 14.

Referring back to FIG. 1, a method of measuring a thickness 26 of anonconductive coating 24 over a portion of the intersection of the firstand second conductive surfaces 20, 22 with a thickness measurement gauge10 is provided. The first step includes placing the eddy current sensor12 in contact with the coating 24 at a predetermined angular orientationwith respect to first and second surfaces 20, 22. The longitudinal axisof measurement 19 of the sensor 12 is at an angle to both first andsecond surfaces 20, 22. The magnetic field generated by the eddy currentsensor 12, as previously described, induces eddy currents in the firstand second conductive surfaces 20, 22. The eddy currents cause anopposing magnetic field. As such, the induced eddy currents are optimum(that is each contributes uniformly to the opposing magnetic field) whenthe measurement axis 19 is substantially at similar angles to eachconductive surface 20, 22. However, if conductivities differ it may bemore advantageous to change the longitudinal axis of measurement 19orientation by placing the axis at a smaller angle with respect to onesurface rather than the other. For example, if the conductivity of thefirst surface 20 is less than that of the second surface 22, placing thelongitudinal axis of measurement 19 at a larger angle (but still lessthan perpendicular) to the first surface may negate some of the effectsof the lower conductivity of the first surface 20 due to its smallercontribution to the opposing magnetic field.

The sensor 12 provides an output based on the impedance change due tothe sum of the magnetic fields. As the sensor 12 is in contact with thecoating 24, the distance from the intersection corresponds to thethickness 26 of the coating 24. The electronic analyzer 40, therefore,provides a relevant output based on that distance. The output may be inelectronic terms, such as millivolts, or calibrated to distancesmeasured, such as microns and millimeters. Therefore, the next step ofthe method includes determining a measurement output of the thicknessmeasurement gauge, which in this embodiment is provided by theelectronic analyzer.

As the output of the thickness measurement gauge has been determined,the thickness 26 of the coating 24 may be determined from the output.The magnitude of the sensed field is proportional to the distance of thesensor from the conductive surfaces. The coating 24 is nonconductive,and, therefore it has a negligible effect on the magnetic fields, and assuch it is only the distance from the intersection of first and secondsurfaces 20, 22 that the measurement gauge is actually sensing.Therefore, in this embodiment, the eddy current sensor 12 is placed incontact with at least a portion of the coating 24. The coating 24 istypically disposed with a concave surface and the sensor 12 cannotcontact all of the coating, thus causing an air gap between portions ofthe sensor 12 and the coating 24. The concavity of the coating 24,however, is known permitting the additional distance due to the air gapto be eliminated from the final thickness determination. Eliminating theair gap consideration may be accomplished by physically measuring theconcavity with a feeler gauge or the like, or, as described in moredetail below, by calibration procedures that compensate for the air gap.

According to one embodiment, the thickness measurement gauge provides ameasurement output that is compared with a predetermined output. Forexample, the predetermined output may correspond to the minimum desiredthickness of the coating. In that case, the predetermined output may belinearly approximated through a range of distances about thatpredetermined output. Therefore, the output may be compared to thelinear approximation to determine the actual thickness.

This embodiment of comparing the measurement output to a predeterminedoutput may be used in conjunction with a pass/fail test. As it isdetermined that the predetermined output corresponds to a minimumthickness and any value less than the output is provided (assuming apositive slope of the output with respect to distance) then any valueless than the output is a fail. Any value greater than the output is apass, if no maximum thickness is required.

The predetermined output may be determined a variety of ways, and onemethod includes calibration to the minimum thickness in a calibrationstand, such as described above. In the embodiments of a thickness gaugedescribed above, the KDM-8200 facilitates such a calibration permittingthe operator to define the linear approximations and predeterminedoutput. Calibration procedures for establishing the desired linearapproximation are provided in KDM-8200 Instruction Manual, Part No.860059-001 Rev. F, the teachings of which are hereby incorporated byreference. Other analyzers include similar specific calibrationprocedures, which will be substituted accordingly.

Referring once again to FIG. 7, a method of calibrating a thicknessmeasurement gauge having an eddy current sensor 12 disposed in a sensorhousing 14 is provided. The sensor 12 is placed at a predeterminedangular orientation with respect to third and fourth conductive surfaces84, 86. The predetermined angular orientation is the same angularorientation that the sensor 12 will be placed relative to the first andsecond conductive surfaces 20, 22 that have a coating 24. The third andfourth conductive surfaces 84, 86 have conductivities that correspond tothe conductivities of the first and second surfaces 20, 22,respectively. Generally, this may mean that the conductivities of thethird and fourth surfaces 84, 86 are equivalent to the conductivities ofthe first and second surfaces 20, 22, respectively. As such, thecalibration procedures most accurately approximate the actualmeasurement.

However, the conductivities of the third and fourth surfaces 84, 86 mayvary with respect to the conductivities of the first and second surfaces20, 22 so long as the proportion of variance is the same for each sothat the variation can be accounted for by calibration procedures,otherwise it is advantageous to ensure that the conductivities aresubstantially equivalent.

As is often the case, the first and second surfaces 20, 22 may be thesame material and conductivity and, therefore, the v-block 82 may alsoinclude third and fourth surfaces 84, 86 comprised of the same materialand conductivity. As such, the v-block 82, as illustrated in FIG. 7, maycomprise a single piece of material and typically, the same material asthe first and second surfaces 20, 22.

The next step of the method includes positioning the sensor 12 to apredetermined distance from the third and fourth surfaces 84, 86. Whenusing a calibration block, this comprises adjusting the positioner 88 sothat third and fourth surfaces 84, 86 are moved toward the sensor holder99. Alternatively, positioning the sensor 12 may also comprise movingthe sensor housing 14 and holder 99 toward third and fourth surfaces 84,86. The predetermined distance from the third and fourth surfaces 84, 86corresponds to the distance to the minimum thickness of a coating 24over first and second surfaces 20, 22. As explained earlier, theconcavity of the coating 24 may require that the sensor be positioned ata distance from the intersection of the first and second surfaces 20, 22that is greater than the minimum thickness. At that predetermineddistance the thickness measurement gauge may be calibrated to apredetermined output.

Referring to FIG. 8, one advantageous method of determining thepredetermined distance is to place a nonconductive coating 104 overthird and fourth surfaces 84, 86. This nonconductive coating 104 may becarefully measured to the minimum desired thickness and havesubstantially the same concavity as is expected from the concavity ofthe coating 24 over first and second surfaces 20, 22. As such, thesensor 12 may be positioned to be in contact with at least a portion ofthe coating 104 in a similar manner to the expected contact with acoating 24 over first and second surfaces 20, 22.

As in the previously described embodiments, calibration may compriseadjusting the linearity, zero, and gain of the electronic analyzer sothat the output is linearly approximated about the distancecorresponding to the minimum thickness. Calibration of the thicknessgauge in this manner may be used in one embodiment to calibrate thethickness gauge to a predetermined output that corresponds to apass/fail test for minimum thickness. The predetermined output isselected so that it corresponds to the minimum thickness, while takinginto account the concavity of the coating. For an output that is apositive slope any greater than the output is a pass.

Therefore, embodiments of a thickness measurement gauge, calibrationstand, and methods of calibrating and measuring with thicknessmeasurement gauge are provided. Many modifications and other embodimentsof the inventions set forth herein will come to mind to one skilled inthe art to which these inventions pertain having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the inventions are notto be limited to the specific embodiments disclosed and thatmodifications and other embodiments are intended to be included withinthe scope of the appended claims. Although specific terms are employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

1. A thickness measurement gauge, comprising: a sensor housing defininga first sidewall on the exterior of the housing such that the firstsidewall permits the housing to abut a first surface proximate to ameasurement area, said sensor housing further defining a second sidewalldisposed at an angle to the first sidewall and configured to permit thesensor housing to abut a second surface proximate to the measurementarea; an eddy current sensor adapted to be electrically interconnectedwith an electronic analyzer, the eddy current sensor being disposed inthe housing and defining a longitudinal axis of measurementcorresponding to magnetic fields generated by the eddy current sensor,wherein the axis of measurement extends through the angle definedbetween the first and second sidewalls at a non-perpendicular angle tothe first sidewall and to the second sidewall; and an electronicanalyzer electrically interconnected to the eddy current sensor andadapted to analyze an electrical signal from the eddy current sensorcorresponding to a distance from a conductive surface.
 2. The thicknessmeasurement gauge according to claim 1, wherein the housing comprises anonconductive enclosure.
 3. The thickness measurement gauge according toclaim 1, wherein the electronic analyzer further comprises a calibrationcircuit.
 4. The thickness measurement gauge according to claim 3,wherein the calibration circuit includes a bridge network electricallyinterconnected to the eddy current sensor.
 5. The thickness measurementgauge according to claim 1, wherein the eddy current sensor is disposedbetween the first and second sidewalls such that the axis of measurementis at a similar angle with respect to both first and second sidewalls.6. The thickness measurement gauge according to claim 5, wherein thehousing defines the first and second sidewalls such that first andsecond sidewalls are perpendicular to each other.
 7. The thicknessmeasurement gauge according to claim 1, wherein the electronic analyzeris independent of the housings.
 8. The thickness measurement gaugeaccording to claim 7, wherein a lead from the housing to the analyzerelectrically interconnects the sensor and electronic analyzer.
 9. Thethickness measurement gauge according to claim 1, further comprising abattery power supply interconnected to the electronic analyzer.
 10. Thethickness measurement gauge according to claim 1, wherein the firstsidewall defines a recessed portion.
 11. The thickness measurement gaugeaccording to claim 1, wherein the first and second sidewalls cooperateto define a beveled corner.
 12. A thickness measurement gauge,comprising: a sensor housing; an eddy current sensor disposed in thehousing; a pressure enclosure hermetically sealed to permit a pressurewithin the enclosure to exceed an ambient pressure; an electronicanalyzer disposed in the pressure enclosure and electricallyinterconnected to the eddy current sensor and adapted to analyze anelectrical signal from the sensor; an electrical connector hermeticallysealed about the pressure enclosure and electrically interconnected tothe electronic analyzer; and at least one lead proceeding from theconnector to the eddy current sensor and electrically interconnectingthe eddy current sensor and the electronic analyzer.
 13. The thicknessmeasurement gauge according to claim 12, further comprising a powersource for energizing the electronic analyzer.
 14. The thicknessmeasurement gauge according to claim 13, wherein the power sourcecomprises a battery disposed within the pressure enclosure.
 15. Thethickness measurement gauge according to claim 13, wherein the powersource is disposed within the pressure enclosure.
 16. The thicknessmeasurement gauge according to claim 12, wherein the pressure enclosurefurther comprises an automatic pressure relief valve.
 17. The thicknessmeasurement gauge according to claim 12, wherein the pressure enclosurefurther comprises a manual pressure relief valve.
 18. The thicknessmeasurement gauge according to claim 12, wherein the housing comprises anonconductive enclosure.
 19. A thickness measurement gauge, comprising:a sensor housing; an eddy current sensor disposed in the housing; apressure enclosure hermetically sealed to permit a pressure within theenclosure to exceed an ambient pressure; an electronic analyzer disposedin the pressure enclosure and electrically interconnected to the eddycurrent sensor and adapted to analyze an electrical signal from thesensor; a power source for energizing the electronic analyzer; and apressure switch electrically interconnected between the power source andthe electronic analyze, the pressure switch adapted to disconnect theelectronic analyzer and power source upon pressure within the pressureenclosure falling below a predetermined threshold further comprising apower source for energizing the electronic analyzer.
 20. The thicknessmeasurement gauge according to claim 19, further comprising a bulkheadfitting through the pressure enclosure pneumatically interconnected tothe pressure relief switch.
 21. A thickness measurement gauge,comprising: a sensor housing; an eddy current sensor disposed in thehousing; a pressure enclosure hermetically sealed to permit a pressurewithin the enclosure to exceed an ambient pressure; an electronicanalyzer disposed in the pressure enclosure and electricallyinterconnected to the eddy current sensor and adapted to analyze anelectrical signal from the sensor; and a pressure gauge for displaying apressure reading corresponding to a pressure within the pressureenclosure.
 22. The thickness measurement gauge according to claim 21,further comprising a bulkhead fitting through the pressure enclosurepneumatically interconnected to the pressure gauge.
 23. A thicknessmeasurement gauge, comprising: a sensor housing; an eddy current sensordisposed in the housing; a pressure enclosure hermetically sealed topermit a pressure within the enclosure to exceed an ambient pressure; anelectronic analyzer disposed in the pressure enclosure and electricallyinterconnected to the eddy current sensor and adapted to analyze anelectrical signal from the sensor; and an output display incommunication with the electronic analyzer for providing a displaycorresponding to the output of the thickness measurement gauge.
 24. Athickness measurement gauge, comprising: a sensor housing; an eddycurrent sensor disposed in the housing; a pressure enclosurehermetically sealed to permit a pressure within the enclosure to exceedan ambient pressure; and an electronic analyzer disposed in the pressureenclosure and electrically interconnected to the eddy current sensor andadapted to analyze an electrical signal from the sensor, wherein thepressure enclosure further comprises an air valve adapted to beconnected to a pneumatic pressure source.
 25. The thickness measurementgauge according to claim 24, wherein the air valve is a schrader valve.26. A thickness measurement gauge, comprising: a sensor housing; an eddycurrent sensor disposed in the housing; a pressure enclosurehermetically sealed to permit a pressure within the enclosure to exceedan ambient pressure; an electronic analyzer disposed in the pressureenclosure and electrically interconnected to the eddy current sensor andadapted to analyze an electrical signal from the sensor, wherein theelectronic analyzer further comprises a calibration circuit.
 27. Thethickness measurement gangs according to claim 26, wherein thecalibration circuit includes a bridge network electrically connected tothe sensor.